1. Field of the Invention
This invention relates to a thin film capacitor device. More particularly, this invention relates to a tantalum-based thin film capacitor device having high performance and providing a high production yield. The present invention also relates to a printed circuit substrate in which such a thin film capacitor device is built as a capacitor, particularly a multi-layered circuit substrate.
2. Description of the Related Art
As is well known, a circuit construction in which a thin film of tantalum (Ta) is formed on a low resistance metal wire such as copper (Cu), the thin film is subjected to anodic oxidation in a suitable electrolyte to form a dielectric layer and the resulting capacitor device is built as a capacitor in a printed circuit substrate, has been employed extensively. To form a capacitor device using the Ta type thin film, for example, fabrication steps that will be explained serially with reference to FIGS. 1A to 1E are generally employed. First, to form a lower electrode made of Ta, a Ta thin film 112 is deposited to a film thickness of about 5,000 angstroms in such a manner as to cover the entire surface of an insulating substrate 111 as shown in FIG. 1A. The insulating substrate 111 may have a protective film on its surface. A sputtering process (hereinafter called also xe2x80x9csputteringxe2x80x9d), for example, can be employed to form the Ta thin film 112. Next, as shown in FIG. 1B, the unnecessary portions of the Ta thin film 112 are patterned, peeled and removed. Next, as shown in FIG. 1C, a resist film 113 is deposited in such a manner as to expose only a dielectric layer formation region of the Ta thin film 112, and then anodic oxidation treatment (which is called also xe2x80x9canode formationxe2x80x9d) is conducted. To conduct this anodic oxidation treatment, the substrate 111 is immersed into a suitable electrolyte and a high voltage of about 280 V is generally applied. As a result, a dielectric layer 114 (comprising tantalum pentoxide Ta2O5) having a film thickness of about 4,500 angstroms is formed as shown in FIG. 1D. Subsequently, the resist film 113 that becomes unnecessary is peeled and removed, and an Au thin film 115 is formed to a film thickness of about 3,000 angstroms by a vacuum deposition process or a sputtering process to form an upper electrode made of gold (Au), for example, as shown in FIG. 1E. Nichrome (NiCr), for example, is preferably formed to a film thickness of about 300 angstroms before the formation of the Au thin film 115, though the NiCr film is not shown in the drawing. Incidentally, this drawing shows the state where the unnecessary portions of the Au thin film 115 are peeled and removed by patterning. There is thus obtained the Ta-based thin film capacitor device comprising the laminate structure of the Ta thin film (lower electrode) 112, Ta2O5 thin film (dielectric layer) 114, and NiCr/Au thin film (upper electrode) 115.
When the Ta-based thin film capacitor device is assembled into the substrate or wiring layer made of Cu, there is the tendency that the Cu ions diffuse into the Ta thin film and positively dissolve into the electrolyte during the anodic oxidation process. This tendency is important. When diffusion of the Cu ions occurs at the time of anodic oxidation, the voltage can be raised to only about 100 V at most as a result of dissolution of the Cu ions into the electrolyte, though the voltage should originally reach a high level (generally, 200 to 300 V). Thus, anodic oxidation cannot be conducted satisfactorily, and pin-holes, unevenness and breakage occur in the resulting anodic oxide film (dielectric layer). Since such defects occur very frequently, the reliability and the yield of the resulting capacitor device drop remarkably, and an increase in the leakage current is induced.
When the surface of the substrate used is coarse, a polyimide resin or an epoxy resin is applied by spin coating, etc., to the surface to planarize the surface, and is set. Thereafter, the Ta thin film is formed. Depending on the condition of anodic oxidation to be conducted subsequent to the formation of the Ta thin film, however, peeling of the Ta thin film occurs due to the stress of the resulting anodic oxide film.
It is an object of the present invention to provide an improved Ta-based thin film capacitor device that solves the problems of the prior art technologies described above, does not invite the drop of reliability and yield, an increase in the leakage current, peeling of the Ta thin film, etc., resulting from anodic oxidation employed during the production process, and has an extremely high adhesion strength between the Ta thin film and the substrate as the base.
It is another object of the present invention to provide a high-performance printed circuit substrate having the thin film capacitor device, provided by the present invention, built therein as a capacitor.
These and other objects of the present invention will be more easily understood from the following detailed description of the embodiments and examples thereof.
According to one aspect of the present invention, there is provided a thin film capacitor device comprising a substrate or wiring layer made of a copper-based low resistance metal, and a dielectric layer made of an anodic oxide of a tantalum-based metal formed on the substrate or the wiring layer, characterized in that a barrier layer comprising at least one kind of a valve metal selected from the group consisting of aluminum, tantalum, niobium, tungsten, molybdenum, vanadium, bismuth, titanium, zirconium, hafnium and silicon, a compound of the valve metal, a mixture of the valve metal compounds, a mixture of the valve metal compound and the valve metal, or an alloy of the valve metal, is interposed between the substrate or the wiring layer and the dielectric layer.
According to another aspect of the present invention, there is provided a printed circuit substrate, wherein the capacitor according to the present invention is built in as a capacitor.